The present invention is a process for making a flowable powdered base suitable for use in the formulation of liquid silicone rubber compositions that can be cured to form silicone elastomers. The process comprises (A) fluidizing a reinforcing silica filler in a high-shear mixer, (B) maintaining the content of the mixer at a temperature below about 60.degree. C. while adding to the mixer a silylating agent, a nitrogen containing compound to facilitate the silylation reaction, and water thereby forming an essentially homogeneous mixture comprising a treated reinforcing silica filler; (C) adding a polydiorganosiloxane having a viscosity of about 0.03 to 300 Pa.s at 25.degree. C. to the mixer and forming a flowable powdered base having an average particle size of from about 1 to 1000 microns, and (D) heating the flowable powdered base under conditions sufficient to remove volatiles.
Curable liquid silicone rubber (LSR) compositions are particularly useful for the injection molding of intricate silicone rubber parts. The LSR compositions offer advantages over convention curable high-consistency silicone rubber compositions, including faster mold cycling times and less deflashing of molded components. Generally, LSR compositions are considered to be those curable silicone compositions which can be pumped by convention pumping apparatuses. Curable LSR compositions usually comprise a polydiorganosiloxane mixture having a viscosity within a range of about 0.03 to 100 Pa.s at 25.degree. C., silica reinforcing filler, and other additives and processing aids.
Curable LSR compositions have traditionally been formed by adding the polydiorganosiloxane, the silica reinforcing filler, and other additives and processing aids to a mixer such as a dough mixer. Normally it takes a dough-mixer from a minimum of about 6 hours to a maximum of 48 hours to form a homogeneous mass having the properties of a LSR that when catalyzed and cured results in a silicone rubber having acceptable physical characteristics.
An objective of the present process is to provide a process which can form a flowable powder base for formulating a LSR composition in a very short time. A second objective of the present process is to provide a flowable powder base which can be further processed easily. A third objective is to provide a flowable powder base which can subsequently be massed to form a curable LSR composition. A fourth objective is to provide a flowable powder base which can be used to formulate curable LSR compositions which can be cured to form silicone rubbers having physical properties at least comparable to LSR compositions prepared by conventional methods.
Link et al., U.S. Pat. No. 3,824,208, describe a process where a silica filler is compounded with a polydiorganosiloxane having a viscosity of between 1 and 200,000 Pa.s at 25.degree. C. to form a free-flowing particulate mixture. Link et al. teach that the filler and polymer may be added simultaneous to the mixing apparatus or the polymer may be added to the mixing apparatus followed by addition of the filler. Link et al. also teach that the filler can be a treated filler.
JP (Kokai) 2/102007 teaches a process where a polydiorganosiloxane having a viscosity of about 100 Pa.s at 25.degree. C., an inorganic filler, and an auxiliary material are evenly dispersed and made into a flowable powder by a high speed mechanical shear method.
Fitzgerald et al., U.S. Pat. No. 4,898,898, describe a process for providing polymer-densified silica fillers. The process is describes as consisting of mixing a silicone polymer having a viscosity within a range from about 1 to 200,000 Pa.s at 25.degree. C. with a reinforcing filler and an extending filler under conditions resulting in a dense, free-flowing particulate mixture.
Bilgrien et al., U.S. Pat. No. 5,153,238, describe forming a flowable powder by blending a high consistency polydiorganosiloxane into a quantity of fluidized reinforcing filler at a temperature within a range of from 100.degree. C. to 200.degree. C. Bilgrien et al. teach the filler can be treated with about 10 to 45 weight percent, based on the weight of the silica, of an anti-creping agent either prior to or during the blending process.
Brown, U.S. Pat. No. 3,268,473, teaches the use of ammonium carbonate or ammonium bicarbonate as an additive to compositions comprising a polydiorganosiloxane, a reinforcing filler, and a hydroxylated organosilicon compound as an anti-creping agent.
Smith, U.S. Pat. No. 3,635,743, teaches an improved reinforcing silica filler can be made by first treating the filler with a nitrogen containing compound selected from a group consisting of ammonia, primary amines, and aminoxy compounds and then treating the filler with a silylating agent.